Links are stored in lists. Documentation on how to work with lists can be found here.
You can access the whole link by issue?.links[0]?.linkedIssue which linkedIssue which returns an Issue object, issue?.links will return a list of objects.
Read
Page properties
id
1
Returned value
The key of the issue which the first link was obtained for, is linked to. If no linked issues are avalibale, null is returned.